Detailed studies of the behavior of captive white-footed mice have cast a number of old problems in new perspectives. Many responses of small captive mammals cannot be interpreted at face value because of severe distortions of behavior that are caused by depriving the wild animal of natural outlets for activity. Confined animals are likely to seize upon and repeatedly exercise virtually any opportunities to modify (and alter their relationships with) their surroundings. In addition they have a strong tendency to counteract nonvolitional and "unexpected" deviations from the status quo. As a result, their responses do not bear an immutable relationship to the nature of the stimulus or other variable being modified; stimuli and activities that are rewarding in certain circumstances are avoided in others. These aspects of behavior have been illustrated by studies of nest occupancy, running in motordriven wheels, and control of intensity of illumination. The results of the control-of-illumination studies suggest the complex interplay of tendencies to modify features of the environment, to avoid conditions imposed compulsorily, and to select preferred levels of illumination. The importance of split-second timing, coordination, and quick reflex actions in the running of activity wheels is indicated by the fact that experienced white-footed mice prefer running in square "wheels" and wheels with hurdles to running in plain round wheels. The relatively conservative behavior of these mice in selecting between multiple sources of food and water and different types of activity wheels suggests the need for careful experimental design in free-choice studies with inexperienced animals. The tendency of trained animals to give some so-called "incorrect" responses even after long experience can be interpreted most reasonably in terms of the adaptive value of a certain degree of variability of behavior in the wild. White-footed mice readily master complex regimes in which several different levers and shutters must be pressed or rotated in certain sequences within seconds for different rewards. They quickly learn to traverse mazes containing hundreds of blind alleys and do so frequently without extrinsic reward. It is unlikely that these remarkable learning performances even begin to approach the capacities of the animals. When two female mice having markedly different solitary behavior patterns were placed in consort, the behavior of each changed, becoming more like that of the other, and the animals showed a strong tendency to remain in each other's company. The behavior of mice in enclosures of great extent casts doubt upon the postulate that hunger and thirst play leading roles in the motivation of wide-ranging locomotor movements. Accordingly, studies of deprived domestic animals in simple mazes may have but limited significance for understanding the behavior of wild and relatively unconfined animals. The existence of marked individual differences between mice selected at random from wild populations sounds the need for a cautio...
A practicable model of the growth process, which gives better definition to the problem of growth and growth regulation and greater precision to related experimental work than do earlier models, is developed on the basis of the following assumptions: "Growth" is the net balance of mass produced and retained over mass destroyed and otherwise lost, implying continual metabolic degradation and replacement. Terminal size represents stationary equilibrium between incremental and decremental components. The mass of an organic system consists of two functionally different components,--generative and differentiated. Generative mass increases by the catalytic action of key compounds ("templates") characteristic of each cell type. Each cell also produces specific freely diffusible compounds antagonistic to these templates ("antitemplates"). Growth regulation occurs automatically by a negative "feedback" in which increasing numbers of antitemplates progressively block the corresponding templates.Differential equations expressing these interrelationships are formulated, integrated, and the solutions evaluated for the case of chick growth. These specific solutions lead to descriptions of the normal growth of a biological system which are in good agreement with known facts, and to predictions of the course of automatic growth regulations after experimental or pathological disturbances which reproduce adequately biological observations in this domain.
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